1. Field of the Invention
The present invention is generally related to mass flow sensors, and more particularly relates to high-performance mass flow sensors made of micro-electro-mechanical systems (MEMS) approach and the methods of making and operating such high-performance mass flow sensors.
2. Description of the Related Art
Conventional technologies of mass flow sensors are still limited by the difficulties of limited ranges of flow rate measurement, low accuracy and the requirement to maintain a high level of heating power. Specifically, the commercially available thermal mass flow sensors are commonly made of transducers that include heaters and temperature sensors. The heater and temperature sensors are commonly provided with resistance wires such as platinum wires on a ceramic substrate. The stream of flow when passing over the mass flow sensor, carries away the heat from the heater thus causes temperature variations. The temperature variations and distributions depend on the mass flow rate, e.g., the velocity and the material properties of the flow. Thus the temperature of the heater and temperature distributions as that measured by the temperature sensors around the heaters can therefore be applied to measure and calculate the mass flow rate.
Heat transfer principle has been widely used for mass flow measurements. Thermal mass flow sensors can be found in many applications from process monitoring, industrial measurements, to medical delivering. In particular, MEMS technology allows fabrication of thermal mass flow sensors directly on silicon with small size, low power, and high reliability at low cost. MEMS-based thermal mass flow sensors have been becoming popular in flow measurement applications. Especially, over the past few years, the advancements made in the technologies of micro-electromechanical system (MEMS) have enabled the fabrication of mass flow sensors directly on silicon. The small size of the MEMS sensor enables new applications of the thermal mass flow rate sensors where size is a key factor. However, as further discussed below, the limited ranges of flow rate measurements and the technical difficulties in improving the measurement accuracy are still the hindrances to broadly apply a MEMS flow sensor in different applications.
Thermal mass flow sensors can be classified into three basic categories: anemometers, calorimetric flow sensors, and time-of-flight sensors. For simplicity, these three types of thermal mass flow sensors are hereinafter abbreviated as A-type, C-type, and T-type mass flow sensors, respectively. Hariadi et al (I. Hariadi, H.-K. Trieu, W. Mokwa, H. Vogt, “Integrated Flow Sensor with Monocrystalline Silicon Membrane Operating in Thermal Time-of-Flight Mode,” The 16th European Conference on Solid-State Transducers, Sep. 15-18, 2002, Prague, Czech Republic) disclose a time-of-flight flow sensor fabricated on Silicon-On-Insulator (SOI) wafers, in which heat pulse is fed to the fluid by a heater and a temperature sensor located downstream detects its delay. Measuring a flight time, the sensors give the velocity of the streaming fluid. However, the pulse will be deformed by the flow velocity profile and broaden at the same time by heat diffusion when it propagates down the stream. This means that the pulse tends to be too broad to be useful for slow flows and thus become inaccurate.
Jiang et al (F. Jiang, Y. C. Tai, C. M. Ho, and W. J. Li, “A Micromachined Polysilicon Hot-Wire Anemometer,” Digest Solid-State Sensors & Actuator Workshop, Hilton Head, S.C., pp. 264-267, 1994) disclose a micro-machined A-type flow sensor comprising of a single element, which is heated and the heat loss of which is measured. This heat loss is dependent on the flow rate of the fluid. This heat loss increases with the flow velocity, and the signal of an anemometer is proportional to the square root of the flow velocity. In general, A-type mass flow sensors are less sensitive in small flows and hence cannot measure small flows accurately. Nevertheless, A-type mass flow sensors have demonstrated that they are capable of accurately measuring flows with high velocities.
Calorimetric flow sensors usually consist of a heater surrounded by temperature sensitive elements arranged symmetrically downstream and upstream. A moving fluid will carry away heat in the direction of flow and accordingly change the temperature distribution around the heater. The temperature difference between upstream and downstream is measured by the temperature sensitive elements. The output signal is commonly fetched using a Wheatstone bridge circuit, in which a pair of downstream and upstream sensing elements comprises two of its four branches. The output signal, which is a measure of temperature difference, is proportional to the flow velocity initially until a high flow velocity is reached where the temperature difference saturates and then decreases at higher flow velocity. In general, calorimetric flow sensors can accurately measure flows with extremely low velocities. However, calorimetric flow sensors may saturate at high flow velocities and hence have a difficulty to measure flows above a certain level of flow velocity.
In summary, there are primary physical limitations for A-type mass flow sensors to extend their measurable flow rate ranges to lower flow velocities. On the other hand, the C-type mass flow sensors is able to extend the measurable flow rate ranges to lower flow velocities but the C-type mass flow sensors become saturated and inaccurate when the flow velocity reaches a higher velocity. Hence, a major concern for mass flow sensors is how to increase their measurable flow rate ranges.
U.S. Pat. No. 4,501,144 describes a calorimetric flow sensor, which was designed to measure either average gas velocity or mass flow rate through a flow channel. This mass flow sensor consisted of two thermally isolated silicon nitride membranes with a central heating, serpentine-resistor-element grid divided equally between the two bridges (or cantilevers). In addition, two identical thin-film serpentine resistor grids with relatively large temperature coefficients of resistance (TCRs) served as temperature sensors, placed symmetrically with respect to the heater on each microbridge. The sensor and heater grids were made of diffused or (temperature-sensitive) thin-film platinum or permalloy (Ni80Fe20), and were encapsulated in a 0.8˜1.0 micron thick dielectric silicon nitride film, which comprised the suspended microbridges. Anisotropic etching of the silicon substrate (with KOH plus isopropyl alcohol) was used to create an air space pit below the microbridges that was preferably ˜125 micron deep, precisely bounded on the sides by (111) silicon planes, and on the pit bottom and ends of the bridges by the (100) and other planes. The symmetry and effectiveness of the micro-bridge that is etched undercut was maximized by orienting the longitudinal axis of each bridge at an angle of 45° with respect to the <110> direction in the mono-crystalline silicon substrate.
In a U.S. Pat. No. 6,550,324, Mayer et al. disclosed a mass flow sensor. As that shown in FIG. 1B, the flow sensor includes a heating element (4) arranged between two temperature sensors in order to measure the mass flow of a liquid or a gas. The mass flow is determined from the temperature difference of the temperature sensors (5, 6). For the pulse of reducing power consumptions, electric pulses are provided to operate the heating element (4). A further reduction of the power consumption is reached by means of a monitoring circuit (12), which switches the actual measuring section (11) on only if the signals from the temperature sensors (5, 6) fulfill a threshold condition. The pulsed power techniques as discussed above still face the difficulties that the range of measurements and accuracy are limited.
However, the above-mentioned techniques as discussed do not provide a resolution to the major concerns for mass flow sensors. Specifically, for those of ordinary skill in the art there is still a need to provide a mass flow sensor to expand the ranges of flow rate with sufficient accuracy.